A sodium ion battery is a battery such that an Na ion moves between a cathode and an anode. Na exists so abundantly as compared with Li that the sodium ion battery has the advantage that lower costs are easily intended as compared with a lithium ion battery. Generally, the sodium ion battery has a cathode active material layer containing a cathode active material, an anode active material layer containing an anode active material, and an electrolyte layer formed between the cathode active material layer and the anode active material layer.
An anode active material used for the sodium ion secondary battery is under active research. For example, in Patent Literature 1, it is described that hard carbon is used as an anode active material for the sodium ion battery. In Patent Literature 2, an anode active material having an Na2Ti6O13 crystal phase is described. In Patent Literature 2, it is described that a carbon material is used as a conductive material, where interlayer distance d002 is 3.5 Å or less or D/G ratio measured by Raman spectroscopy measurement is 0.8 or less. Meanwhile, in Patent Literature 3, an Nb oxide is disclosed as an active substance for insertion and desorption of an Li ion, not an Na ion.